Failover Management and Issues in .NET

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What if a failure occurs late at night in the middle of a long holiday weekend What happens when knowledgeable administrators abruptly leave the company How quickly can they be replaced and their replacements trained What happens during the interim What happens when the satellite that relays pages to pagers gets knocked out of orbit, as happened in 1996. (The resulting outage lasted several days.) What happens when procedures are not followed properly, and as a result, systems get damaged worse than before the alert What happens when your knowledgeable people cannot gain access to the servers, or cannot find the servers when a failure has occurred
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Automatic failover software addresses these issues. It will respond automatically to a failure and respond in preprogrammed ways. It knows which host has failed and which hosts are eligible to take over for it. It can be used to identify loads across nodes in a large cluster and to determine which node is best able to handle the additional load that the downed resources will require.
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Another option for implementing an FMS is to write it in-house. In our experience, homemade FMS consists of code that one or two system administrators wrote in their copious spare time to manage failover for one or two particular pairs of servers. Since their salaries are preapproved expenses, it may seem that having them write this code is a wonderful idea, as the project looks like it is free. But who is left doing the job the administrators were hired to do while they are writing and testing this code But putting that aside, let s compare the likely results of this coding effort to our key HA design principles: Homegrown FMS is not mature code. Nobody else has tested it. You are placing your critical systems under the care of code that has never been rolled out anywhere else and has never recovered a production application or system before. Homegrown FMS is version 1.0 code. As it is new and untested, it will inevitably have bugs. The only way to discover the bugs is to use the code. Testing the code in production violates another principle. Homegrown FMS has no reference sites. It also has no USENET newsgroups to discuss problems and issues with other users, or any user
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conferences, magazines, books, or any of the other ancillary features that mature, widely installed code offers. Homegrown FMS has little or no support. Whom are you going to call for help when it doesn t work on a weekend or in the middle of the night What happens when the authors leave the company They take the support with them, and you are left with none. Your homegrown FMS may handle the easy failure cases well, but there are many corner cases that do not occur very often. Nevertheless, these oddball cases must be handled by any worthwhile FMS. The developers must first think of them, and then write code to address them. Since they seldom occur, these cases will probably not be adequately tested; when they do occur, can we be confident that the code will handle them properly Decisions must be made in advance regarding the best way to handle these situations. Commercial FMS solutions have examined these and have either worked out ways to handle them (hopefully the best ways to handle them) or have given the user configuration options that he can choose from at install time. The following are some examples of corner cases:
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What if a clustered host loses connectivity to the public network What if a clustered host loses all of its heartbeat links How can we be sure it has really failed What if an application is running and can read cached data from memory, but cannot access certain disks or data What happens if both servers in a failover pair boot at the same time Which takes charge Does it matter What if the takeover server has problems and cannot accept a failover when it is expected to What happens if a failover script has an error in it and cannot run to completion What if a server is too busy to send heartbeats for awhile What happens if a few heartbeats get corrupted on their way to the other server What if the FMS daemon crashes or is manually killed How do you handle extreme levels of network or system latency What happens if the Name Service goes away and your software can t resolve IP addresses What happens if the routing tables get messed up and heartbeats start routing over the public network Or worse, what happens if public traffic goes over the heartbeat network
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